The presently disclosed subject matter relates to a medical photometer which calculates the amount of a material having a relatively low extinction in the body of the subject, and also to a method of controlling a medical photometer for calculating the amount of the material.
A pulse photometer that is an example of a medical photometer is an apparatus which calculates the concentration of a blood light absorber of the subject as an example of photometry.
Specifically, the living tissue of the subject is irradiated with light beams at a plurality of wavelengths at which ratios of the blood extinction coefficients are different from each other depending on the blood light absorber concentration. The intensities of the light beams at the wavelengths transmitted through or reflected from the living tissue are detected. The intensities at the wavelengths are varied in accordance with the pulsation of the blood in the subject. Therefore, due to the pulsation, temporal variations of the intensities at the wavelengths are acquired in the form of a pulse wave signal. The amplitudes of pulse wave signals with respect to wavelengths correspond to light attenuation variations with respect to the same wavelengths. The blood light absorber concentration is calculated based on a ratio of light attenuation variations with respect to wavelengths (for example, see Japanese Patent No. 4,196,209).
There is a need for knowing the amount of water or adipose tissue among materials existing in the body of the subject. However, these materials have an extinction which, as compared with the above-described blood light absorbers, is lower over the whole wavelength range (for example, 190 to 1,100 nm) where a general-purpose optical sensor is sensitive. In the specification, such a material is referred to as “weak light absorber in body.”
It is an object of the presently disclosed subject matter to calculate the amount of a weak light absorber in body even with the principle of photometry.